Packing and cracking are the two techniques behind almost every gerrymander. Mapmakers use them to turn the same set of voters into a very different set of election results, simply by deciding where the district lines fall. This guide explains how each technique works, walks through a simple example, and shows how to recognize them. To see the lines that affect you, you can look up your own congressional district by address or ZIP code.
What packing means
Packing concentrates as many of the opposing side’s voters as possible into a small number of districts. Those districts are then won by enormous margins, often 70 percent or more. The side being packed still wins those seats, but every vote above the 50 percent needed to win is, in effect, a wasted vote. Because all of that voting strength is bottled up in a few districts, it cannot help win any of the surrounding seats.
What cracking means
Cracking is the opposite move. Instead of bunching a group together, it splits that group across many districts so that it falls just short of a majority in each one. A community that could have elected a representative of its choice in one compact district is divided three or four ways, leaving it outvoted everywhere. Cracking dilutes voting strength rather than concentrating it.
A simple worked example
In a state with ten districts and an electorate split 60 to 40 between Party A and Party B, fair lines would give Party B around four of the ten seats. A mapmaker who favors Party A can do better. By packing Party B’s strongest neighborhoods into two districts that Party B wins with 80 percent, and cracking the rest of Party B’s voters thinly across the other eight districts so they top out near 45 percent in each, Party A can win eight seats to two. Party B’s statewide share barely changed, but its share of seats collapsed.
Wasted votes and the efficiency gap
Both techniques work by creating wasted votes, meaning votes that do not contribute to electing a candidate. A wasted vote is any vote for a losing candidate, plus any vote for a winning candidate beyond the threshold needed to win. Packing wastes the opposing side’s surplus votes; cracking wastes its losing votes. The efficiency gap, a measure used by political scientists and sometimes by courts, compares how many votes each side wastes. A large, one-sided gap is a signal that a map may be drawn for advantage.
How to spot packing and cracking
You usually cannot tell from a single district whether a map is gerrymandered, but a few patterns are worth watching. Look for districts with strange, sprawling shapes that reach out to grab or avoid particular neighborhoods. Compare win margins across a state: a mix of a few blowout districts for one side and many narrow wins for the other can be a fingerprint of packing and cracking working together. Communities that are repeatedly split across district lines are another clue.
Both sides use these tools
Packing and cracking are not unique to one political party. Whichever party controls the map-drawing process has the opportunity and the incentive to use them, which is why some states have shifted map-drawing to independent or bipartisan commissions. The techniques themselves are neutral tools; what matters is who is drawing the lines and toward what end.
See the lines for yourself
The legal line between race and party
Not all packing and cracking is treated the same in court. Using these tactics to weaken the voting power of a racial or language minority can violate the Voting Rights Act, and federal courts can strike down such maps. Drawing lines for purely partisan advantage is different. In 2019 the Supreme Court ruled that federal courts cannot hear partisan gerrymandering claims, leaving that question to the states and state courts.
What reformers propose
People who want to limit packing and cracking push several changes. The most common is handing map-drawing to an independent commission rather than the legislature. Others call for rules that favor compact districts or that keep counties and communities together. Researchers have offered measures such as the efficiency gap to flag lopsided maps, though the Supreme Court declined to adopt such a test for federal claims. Reform mostly advances state by state.
Why it is hard to stop
Packing and cracking persist because both parties use them when they control the process, so neither has a strong incentive to give up the advantage. Proving that a map crossed a line is also difficult, since population and geography can explain some odd shapes. Detailed voter data and mapping software now let map-drawers fine-tune districts with great precision, which makes careful gerrymanders easier to build and harder to detect.
The best way to understand these techniques is to look at real districts. Read our overview of what gerrymandering is, learn how congressional redistricting works, then find your district by address or ZIP or browse all 435 congressional districts to see how your own boundaries are drawn.
Last updated August 12, 2026.
